Thimerosal, (sodium ethylmercurithiosalylate, (Merck 8th Ed. pg. 1040) ##STR1## is a well known antiseptic for surface tissues employed in a dilute aqueous solution or in a tincture. In veterinary use it is employed as a wound antiseptic and in mycotic skin infections. It has been discovered that thimerosal has anti-viral properties. In vitro studies have shown that it is quite toxic to herpes simplex and in vivo studies with rabbits have shown it to be more effective than idouridine (IDU) (Merck, 8th Ed. pg. 561), a recognized anti-viral agent for herpes simplex keratitis. In addition, the thimerosal solution has no adverse effect on the epithelium where as IDU does exhibit such effects.